With the governing Socialists and other left-wing parties enjoying a strong majority, the new law is likely to sail through the first reading debate and gain final approval before a visit by Pope Benedict XVI, due in Portugal in May.

In contrast to Spain, where the lead-up to the legalization of same-sex marriage in 2005 brought hundreds of thousands of demonstrators onto the streets, the bill in Portugal has provoked only muted opposition even from the right.

While normally vocal on the role of marriage and the family in society, the Catholic Church has refused to mobilize on a subject which, according to Lisbon's Cardinal Patriarch Jose Policarpo, is "parliament's responsibility."

"I think the Portuguese people have learnt one of the fundamental tenets of democracy: respect for the rights of the individual," Miguel Vale de Almeida, Portugal's first openly-gay lawmaker who was elected in September, told AFP.

Vale de Almeida, who is the Socialists' pointman on the legislation, said there is now a political majority in favor of same-sex marriage and that it is "too simplistic to link Catholicism and conservatism."

According to poll conducted late last year by the Eurosondagem institute, while a strong majority (68.4 percent) of Portuguese are opposed to adoptions by same-sex couples, they are more evenly divided when it comes to same-sex marriage with 49.5 percent against, with 45.5 percent in favor.

On Tuesday, campaigners handed a petition with more than 90,000 signatures to demand a referendum on the subject into parliament.

But having had its fingers burnt by two referendums which preceded the legalization of abortion in 2007, the government has ruled out consulting with the public as the measure was part of its manifesto in last year's election.

Prime Minister Jose Socrates' Socialists may have lost their majority in the September 27 election, but still command the support of other left-wing parties in parliament who should guarantee that the marriage bill is passed.

While opposed to the concept of same-sex "marriages", the center-right opposition Social Democrat party says it is favor of a civil partnership that would give gays and lesbians the same rights as heterosexual couples minus adoptions.

Deputies are also expected on Friday to vote two other bills submitted by the Green party, the Left Bloc and others which would grant gay and lesbian couples the right to adopt children.

If the marriage proposals do pass through parliament, they will the have to go through a parliamentary commission before coming back for the final approval.

According to media reports, both the government and the Catholic Church wants the gay marriage issue to be resolved before the visit of the pope, scheduled for May 11-14.